1. Field of the Invention
The present invention relates to a sputtering apparatus for supplying substantially constant DC power to a sputtering load having impedance that varies over a wide range.
2. Description of Related Art
This type of DC sputtering apparatus is used, for example, as a thin-film forming device, in which case an inert gas such as argon is introduced into a vacuum chamber and a negative voltage of several hundred volts is impressed on a target electrode made of aluminum, copper, or titanium to generate a plasma discharge. The plasma discharge positively ionizes the inert gas, and the positive ions are then accelerated toward and collide with the surface of a target. This causes the target material to be vaporized, with the vaporized material being deposited on the substrates of, for example, semiconductor surfaces and optical disks, to form a thin film comprising the target material on the substrates.
In this way, in a sputtering apparatus that generates a plasma discharge in a gas (or vacuum) from a relatively low voltage, the sputtering voltage changes significantly depending on the target material and type of introduced gas. For example, a typical sputtering voltage changes from 500 V to 1,000 V, a nearly twofold voltage range, with the same rated power required at each voltage within the range. That is, while the impedance of the sputtering load varies over a fourfold range, constant power must be supplied to the sputtering load whose impedance varies over this wide range.
FIG. 7 shows an ideal case of the output characteristics for a 10 kW sputtering apparatus whose load impedance quadruples from 25 Ω to 100 Ω. With a rated voltage of 1,000 V, the current available to a negative load of 100 Ω is 10 A, and with a voltage of 500 V, a 20 A current can be supplied to a negative load of 25 Ω. FIG. 7 also shows that this type of device requires a rise in the voltage for a current near zero. This is due to the need to apply an initial trigger voltage of at least 1.5 times the rated voltage in order to start the plasma discharge in the sputtering apparatus.
However, in the design of a normal power supply device that delivers 1,000 V×10 A of power, the maximum current available at 500 V is also around 10 A, even allowing for small discrepancies arising from the circuit configuration. For this reason it is necessary to design a large capacity power supply device that delivers 1,000 V×20 A of power in order to supply 10 kW at both 1,000 V and 500 V. However, a converter designed for excessively high power increases the reactive current of the converter circuit, thereby generating significant loss.
FIG. 8 shows an example of a conventional DC sputtering apparatus. DC power supply 41 is provided with, for example, a three-phase bridge rectifier that rectifies a three-phase AC power supply. The DC voltage is then converted to a high frequency AC voltage by inverter 42. This high frequency AC voltage is converted to the appropriate voltage for sputtering by transformer 43. Secondary winding 432 of the transformer 43 is provided with center tap 435 in addition to end taps 433, 434, with taps 434 and 435 switchable by tap switch 436 depending on the required voltage. Connection to the center tap 435 is shown in the drawing. The AC voltage from the selected tap is input to the AC input terminal of bridge rectifying circuit 44, which converts the transformed high frequency voltage to DC voltage. Filter condenser 45 then supplies a negative electrode voltage with reduced ripple voltage to sputtering load 46. Housing 47 of the sputtering load 46 is connected to the positive electrode side of the rectifying circuit 44 and grounded Transformer 43 is provided with a function that insulates the commercial power source polarity and the sputtering load 46.
In such a conventional sputtering apparatus, either tap 434 or 435 is selected in advance depending on the impedance level of the anticipated sputtering load in accordance with the gas introduced to the sputtering load 46 and the target material. For example, for materials with a low impedance load not exceeding 600 V, the center tap 435 is selected, while when a high impedance load of around 900 V is expected with a change in film forming conditions, the end tap 434 is selected, as shown in the figure.
However, the rise in the gas temperature following the onset of discharge alters the impedance of the sputtering load 46 and the voltage changes. When a tap not matching the changed impedance is selected, a problem arises in that the voltage enters a range in which the rated power is not attained, thereby inhibiting the specified sputtering process.
In addition, switching taps is an extremely troublesome procedure in which, to ensure safety, the tap is changed only after interrupting the AC voltage fed into the DC power supply 41, removing the cover of the power supply, discharging the residual charge of the condensers in the circuit, and confirming that discharge is complete While it is possible to switch taps by employing a switch external to the power supply device, this has the drawback of leading to a cost increase of the power supply device due to the added complexity and higher cost of its structure, particularly its insulation structure and contact structure.
A related document is Japanese Unexamined Patent Application, First Publication No. 2001-335928.
Accordingly, it is an object of the present invention to provide a sputtering apparatus that can supply a rated power to the sputtering load over a wide impedance range without using a transformer tap.
It is a further object of the present invention to provide a sputtering apparatus that automatically matches even large changes in the impedance of the sputtering load to supply the rated power.
It is yet another object of the present invention to provide a sputtering apparatus that can supply the rated power to a sputtering load whose impedance varies as widely as up to a fourfold range.